mpc83xx: Migrate CONFIG_SYS_{BR, OR}*_PRELIM to Kconfig

Migrate the BR/OR settings to Kconfig. These must be known at compile
time, so cannot be configured via DT.

Configuration of this crucial variable should still be somewhat
comfortable. Hence, make its fields configurable in Kconfig, and
assemble the final value from these.

Signed-off-by: Mario Six <mario.six@gdsys.cc>